1. Field of the Invention
The present invention relates to fiber-optic communications systems, such as telecommunications systems and data communications systems.
2. Description of the Related Art
In today""s marketplace, as the demand for high-rate communications systems continues to increase, commercial communications system designers continue to attempt to find cost savings and technological improvements in components, performance efficiencies, and transmission rates. To address these system goals, communications system designers have implemented designs and solutions using optical fiber as a transmission medium that exhibits high bandwidth and low transmission loss. Traditional fiber-optic communications systems exhibit superior data rates and demonstrate rapid recovery of capitalized costs. As used herein, the term xe2x80x9coptical fiberxe2x80x9d has its accustomed meaning of a fiber (e.g., a thin rod shape) containing one or more core regions within which light travels, and, in certain fabrications, a cladding layer outside of the outermost core region.
For decades, optical fiber has been the preferred transmission medium in high-capacity communications networks and long-distance communications systems. More recently, optical fiber has also begun to be used in short-distance communications applications, such as local area networks, integrated applications, and intra-system applications. In general, long-distance applications have communications links greater than about 1 km, while typical short-distance applications have communications links shorter than about 1 km, and often as short as tens of meters.
Typically, optical implementations of short-distance communications systems employ glass optical fiber that generally satisfies the desired physical and performance characteristics previously discussed. In particular, existing glass optical fiber exhibits low transmission losses, generally less than 1 dB/km. As used herein, the term xe2x80x9cglass optical fiberxe2x80x9d refers to any suitable silica-based fiber with or without optical doping impurities.
For example, a technically superior solution for making short-distance links is known to be based on single-mode glass optical fiber. This type of fiber offers exceptionally high bandwidth, since it supports only one propagating electromagnetic mode. Typically, the optical core of a single mode fiber is only several microns in diameter. Consequently, connecting to the optical core of the fibers requires precision optical and mechanical couplings that significantly increase the material and labor cost of the links.
Attempts to reduce the high costs associated with single-mode glass optical fiber systems include the approach of implementing links based on multimode glass optical fiber. Multimode glass optical fiber possesses a larger core diameter than single-mode fiber, generally in the range of approximately 50-62.5 microns. Thus, the mechanical and optical couplings required for multimode fiber connections are generally less precise, and therefore less expensive to manufacture and install, than those used for single mode fibers. However, the resulting bandwidth of multimode fiber is degraded by dispersion of the various electromagnetic modes which propagate in the fiber. While fiber manufacturers attempt to fabricate multimode fibers with an index profile that minimizes this intermodal dispersion, manufacturing limitations often result in non-ideal index profiles and associated degradation of bandwidth.
To further reduce the cost of optical links, the prior art also includes the use of multimode plastic optical fiber. Since plastic is less brittle than glass, plastic fibers can have significantly larger cores than multimode glass fiber, thereby allowing even further relaxation in the mechanical and optical tolerances of couplings at fiber endpoints. These relaxed tolerances further lower the cost of the optical transceivers and couplings in the links. Also, since methods exist to terminate plastic fiber endfaces with very little skill and effort, the cost required to install plastic fiber links may also be lower than comparable links using glass fibers. Although multimode plastic fibers offer simplicity and lower cost, serious limitations exist in the technologies for fabricating these fibers. As a result, plastic fibers are usually produced with either a step refractive index profile or a graded refractive index profile that differs significantly from that required for maximum bandwidth.
One way to overcome the bandwidth limitations of glass and plastic multimode fibers is to introduce nonuniformities during manufacture, such that power diffuses between one or more pairs of the electromagnetic modes of the fiber. The presence of such nonuniformities is referred to herein as xe2x80x9cmode coupling.xe2x80x9d Mathematically, the diffusion of power between a pair of modes (labeled i and j) may be described with the differential equation dPj(x)/dx=CijPi(x), where x is the spatial coordinate parallel to the fiber axis, Pj(x) is the amount of power in the jth mode at point x, Pi(x) is the amount of power in the ith mode at point x, and Cij is the coupling constant between modes i and j. A fiber of length L will be considered herein to be xe2x80x9cmode coupledxe2x80x9d if any of the coupling constants Cij are sufficiently large to produce a significant change in modal power distribution as an optical signal traverses the length of the fiber. A fiber that does not meet this condition is referred to herein as a xe2x80x9cstandardxe2x80x9d multimode fiber. Since the coupling constants discussed above are often unknown in practice, a simpler, but less precise, definition of mode coupling strength will be introduced below. Note that a mode coupled fiber with a given set of coupling constants will cease to be mode coupled if it is cut to a sufficiently short length. By the same token, fiber that operates as a standard multimode fiber at one length will be mode coupled at a sufficiently long length.
In mode-coupled fibers, photons injected into the fiber sample many of the various electromagnetic modes while transiting the fiber. As a result, they arrive at the output end of the fiber with a narrower distribution of arrival times than they would in the absence of such mode coupling. The net result of this mode coupling is to reduce the effective intermodal dispersion of the multimode fiber, thereby increasing fiber bandwidth. This phenomenon has been well documented in both glass and plastic multimode optical fibers, and can be conveniently used to parameterize the strength of the mode coupling. Herein, if a multimode fiber has an index profile such that its monochromatic bandwidth would be Bo in the absence of mode coupling, and if mode coupling acts to increase the observed monochromatic bandwidth to a value Bc, then the mode coupling will be said to be of strength F=Bc/Bo.
While mode coupling improves the bandwidth of multimode fibers, it also increases their attenuation compared to a comparable standard multimode fibers. Many types of mode coupling non-uniformities result in an additional loss that increases quadratically with mode coupling strength. Mathematically, the excess loss xcex1c due to mode coupling is xcex1c=F2*0.5 dB/km.
While a significant body of prior art teaches methods for creating mode coupling in optical fibers, this knowledge has found little practical application, due to the increased loss that accompanies mode coupling. Historically, optical fibers were developed for use in long-distance links, where the large length scales involved made minimal fiber attenuation imperative. Since even a modest bandwidth improvement, say F=2, necessitated 2 dB/km excess loss, such fibers were not employed. In later years, when multimode optical fibers began to be significantly used for short-distance links, systems were designed with an assumption that the fiber medium would exhibit the very low losses achieved for long-distance transmission. Thus, existing link designs allow very little budget for attenuation in the optical fiber. Even with these low attenuation budgets, the maximum length of optical links using standard multimode fibers is usually limited by fiber dispersion, not by attenuation. Because existing short-distance systems allow very low budgets for fiber attenuation, designers have not contemplated short-range transmissions system using very heavily mode-coupled fiber. As a result, short-distance multimode optical transmission systems continue to be limited almost entirely by intermodal dispersion.
The present invention is directed to short-distance fiber-optic communications systems, where the systems are designed to use fibers with stronger mode coupling by recognizing and compensating for the generally differing and unique characteristics of such fibers in comparison to those of standard multimode optical fiber. Accordingly, these communications systems are configured to compensate for the shortcomings of mode-coupled optical fibers, while preserving their high bandwidth. More advantageously, such systems will also reduce the complexities (and associated costs) involved in the optical and mechanical couplings between active devices and optical fibers.
Primarily, the present invention provides communications systems using optical fibers with much stronger mode coupling and higher launched optical power when compared to existing systems using glass optical fibers. The present invention further provides these systems with a receiver having a greater dynamic range when compared to existing short-distance optical fiber systems. The present invention further provides for implementation of these systems based on mode-coupled fibers comprised of either plastic or glass. The present invention further provides that these fibers may have core diameters in the range of existing standard multimode optical fibers (approximately 50-62.5 microns), or more advantageously, may have significantly larger core diameters.
The present invention provides communications systems that utilize mode-coupled optical fibers at comparable or lesser expense than systems using standard multimode optical fiber. The present invention provides communications systems using mode-coupled optical fibers that provide for data communications at both high and low rates. In general, the fiber medium employed in the system is chosen to have a high degree of mode coupling, and the power coupled from an optical transmitter into the fiber is designed to be higher than an analogous system comprised of standard glass optical fiber. The increased launch power is proportionate to the loss budget allocation and overcomes the higher attenuation characteristics of the mode-coupled optical fiber. In addition, the dynamic range of the preamplifier portion of an optical receiver is chosen to be of a range proportionate to the launch power, such that saturation of the preamplifier does not occur. More advantageously, the spatial dependence of the dispersion characteristics of mode-coupled fiber is also exploited to achieve simplified optical and mechanical couplings compared to glass fiber systems.
The present invention may be implemented using mode-coupled optical fiber of either graded-index or step-index type, wherein the fibers are made from a glass, perfluorinated or substantially fluorinated plastic, or protonated plastic material.
In one embodiment, the present invention is a fiber-optic communications system having an optical link comprising (a) a transmitter; and (b) a receiver, coupled to receive optical signals transmitted by the transmitter over a mode-coupled optical fiber having a mode-coupling strength F of about 2 or greater. The transmitter is configured to transmit the optical signals at a launch power level that takes into account a fiber attenuation budget of about 5 dB or greater; and the receiver has a dynamic range that is selected based on the fiber attenuation budget.